1. Field of the Invention
This invention relates to articles capable of being heated by microwave energy.
2. Discussion of the Prior Art
The use of microwave radiation to generate heat in materials is becoming increasingly more prevalent in both consumer applications and in industrial applications because of the increased speed and lower power consumption of microwave processes, when contrasted with conventional heating processes. Microwave susceptors are materials that absorb microwave energy, convert the absorbed energy to heat energy, and thereby heat surrounding media. Two general categories of susceptors are: (1) thin-film susceptors; and (2) particulate susceptors.
Thin-film susceptors typically comprise a metal coated polyester film laminated to a substrate such as paper or cardboard. The substrate serves to increase the dimensional stability of the susceptor during use in a microwave oven. The use of such susceptors is well known in the art and is described in a number of patents, such as, for example, U.S. Pat. Nos. 4,267,420; 4,230,924; and 4,641,005; and Canadian Patent No. 1 153,069.One example of a thin-film susceptor involves the application of electrically resistive films to the surface of an article (e.g., a dish) to convert microwave energy to heat by means of so-called I.sup.2 R or resistive loss (see, for example, U.S. Pat. No. 3,853,612). The disadvantages associated with conventional thin-film susceptors include rapid decline in heat output of the susceptor (prohibiting reuse of susceptors), arcing, difficulty in regulating heating rate, and the necessity of a separate processing step to laminate the film onto a substrate.
Particulate susceptors can be conveniently classified into two groups, namely electrically continuous particulate susceptors and electrically discontinuous particulate susceptors. In a matrix that contains electrically continuous particulate susceptors, the particles are in sufficiently close proximity to each other such that the matrix will conduct electricity. Heat is generated in the matrix by the interactions between adjacent particles, such as arcing and eddy currents.
Commonly used particulate materials for electrically continuous susceptors include carbon black, graphite, and finely divided metal particles. One such susceptor is described in U.S. Pat. No. 4,518,651, in which carbon black is loaded into a polymer matrix up to a level of 60% by weight carbon black. U.S. Pat. No. 4,640,838 discloses a self-venting, vapor-tight package suitable for use in a microwave oven having a deposit comprising non-metallic microwave-absorbing particles such as graphite dispersed in a non-metallic binder. When the package is heated in a microwave oven, heat build-up in the particles may soften and weaken the underlying packaging material, thereby venting the package. U.S. Pat. No. 3,701,872 discloses the incorporation of resistive particles within a body to be heated by microwave energy, such that a plurality of electric arcs are generated throughout the particles, thereby resulting in microwave energy being converted into heat energy. The disadvantages of electrically continuous susceptors include non-uniform heating, the possibility of catastrophic arcing, and difficulty in regulating the heating rate. Additionally, when metal particles are used, the level of loading that is required will result in a high density, and consequently a heavy susceptor.
Electrically discontinuous susceptors can comprise metal, semi-conductor, and/or ferromagnetic particles, which are dispersed or positioned in a matrix to be heated by microwave energy. The particles impart a lossy nature to the composite so that it can be heated by microwave energy. The absorption of microwave energy and the subsequent generation of heat occurs in isolated particles rather than through interactions of adjacent particles. These types of susceptors are described in, for example, U.S. Pat. Nos. 4,226,108; 4,362,917; and 4,450,334. Although these susceptors do not decline substantially in heat output and give fairly controllable heating, their disadvantages can include high density (heavy susceptor), high expense, and non-uniform dispersion.
U.S. Pat. No. 2,830,162 discloses that ferrite particles can be embedded in the material for making the body of a microwave browning dish to give the dish an upper temperature limit, resulting from the Curie effect. The Curie effect causes the microwave energy absorption capability of ferrite particles to vary with temperature. As the temperature increases, less microwave energy is absorbed, resulting in a lesser rise in temperature.
U.S. Pat. No. 3,585,258 discloses the use of divided iron or divided carbon as materials for converting microwave energy to heat energy for use in a microwave kiln to fire ceramic articles. According to this patent, the ceramic article to be fired is either placed near the divided material, filled with the divided material, or buried in the divided material.
Inorganic thin-film coatings, especially of metals, have long been applied to finely divided particulate matter for a variety of purposes. Materials for thin-film coatings are disclosed in U.S. Pat. No. 4,618,525.